def listf(args):
    return args

def empty(args):
    return len(args[0]) == 0

def car(args):
    assert len(args) == 1, "car takes one arg"
    if args[0]:
        return args[0][0]
    else:
        return None

def cdr(args):
    assert len(args) == 1, "cdr takes one arg"
    return args[0][1:]
    
rename = {'listf':'list','empty':'empty?'}
